1. Field of the Invention
This invention relates to a color composition for a color filter, to a color filter where this color composition is employed, and to a liquid crystal display device which is provided with this color filter.
2. Description of the Related Art
In recent years, thin display devices such as a liquid crystal display device are increasingly demanded to enhance the picture image and power-saving thereof and to lower the manufacturing cost thereof. Therefore, in the case of the color filter to be employed in such display devices, it is desired to employ a photosensitive color composition which is sufficiently high in color purity, in brightness and in contrast and makes it possible to produce a fine pattern accurately and at a low cost while developing the double refraction in conformity with various kinds of liquid crystal display modes.
Especially, with respect to the viewing angle characteristics of liquid crystal display device, it is now demanded to exhibit a very high level of display quality. More specifically, on displaying black color of liquid crystal (in the case of normally black display, the black color in field-off state), it is demanded to realize pure black color and to display this pure black color at a wide viewing angle. For this purpose, various ideas have been practiced attaching importance to the features of a retardation film, to the method of aligning the liquid crystal molecules and to the method of driving the liquid crystal molecules. Since the display of “black” to be dealt therewith is directly related with the value of contrast ratio on the occasion of ON/OFF of driving signals, the display thereof is important in this respect.
Further, there is such a high level of demand for the “black display” that even if there is a minute retardation (a retardation in thickness direction) which is caused to generate by each of color pixels constituting a color filter to be formed through the dispersion of organic pigments, it is demanded to make sure that the black display can be prevented from being badly influenced. In order to meet this demand, various methods have been tried to reduce the quantity of retardation that the color filter may exhibit, the methods including one wherein a macromolecule having a planar structural group on its side chain is introduced into a color layer constituting a color pixel, or one wherein a double refractive index-reducing particles having a double refraction which is opposite in sign to that of a macromolecule is introduced into a color layer constituting a color pixel (see for example, JP-A 2000-136253 and JP-A 2000-187114).
Further, there has been proposed an idea to incorporate a retardation-adjusting agent in the color layers of color filter, thus enabling each of subpixels to have a different retardation, thereby making it possible to enable the viewing angle compensation of black state of a liquid crystal display device to be effected in the wavelength of almost all visible light zone without necessitating the provision of a polymeric liquid crystal layer in addition to the color layers or without necessitating the change of thickness in each of subpixels (see for example, JP-A 2008-20905 and JP-A 2008-40486).
However, in the cases of the prior art wherein the double refractive index-reducing particles or the retardation-adjusting agent is employed, even if it is possible to adjust the retardation of a color polymer film formed a coated film, it has been found impossible to optimize the characteristics of color resist employed as a starting material for forming color pixels. Specifically, it has been found difficult to concurrently optimize not only the developing properties of a color resist at the step of photolithography, the adhesion of the color resist to a substrate and the storage stability of the color resist but also the adjustment of retardation.
Incidentally, it has been found out by the present inventors that the value of retardation in thickness direction which the color layer may exhibit greatly differs depending on the kinds of pigment to be employed and that the magnitude of the value of retardation in thickness direction becomes larger depending on the pulverization or dispersion of the pigment or on the kinds of matrix resin (for example, acrylic resin, cardo resin, etc.).
For these reasons, in the case of the conventional methods where double refractive index-reducing particles or the conventional retardation-adjusting agent is employed, even if it is possible to increase the value of retardation in thickness direction in the positive direction, it is difficult to shift the retardation of color layer existing in a plus region toward less than +2 nm or in the negative direction which is required for obtaining “black” of a higher level.
Meanwhile, with respect to the demand for contrast, although there has been proposed an idea of using acrylic resin comprising, as essential components, benzyl (metha)acrylate and (metha)acrylic acid (see for example, JP-A 10-20485), it is still impossible to realize the development of double refraction in conformity with each of various liquid crystal display modes.